1. Field of the Invention
The present invention relates to a bias generation circuit and a voltage controlled oscillator that are suitable for a large-scale integrated circuit for radio communication.
2. Description of Related Art
Conventionally, in an LSI for radio communication, various bias voltages (or currents) are supplied to the circuit. For example, in order to decide the operating point of a transistor, a bias voltage is applied to a gate of the transistor. Further, for example, a bias current is supplied by a current source to a common source of transistors that configure a differential amplifier. A bias generation circuit that generates this kind of bias can be composed by variable resistors that divide a power supply voltage.
Variations in the characteristics of transistors occur due to temperature changes, fluctuations in the power supply voltage, or the influence of variations in manufacturing. It is possible to reduce the influence of such variations in the characteristics by adjusting a bias that is supplied to a circuit. However, in a bias generation circuit of a simple configuration that uses only variable resistors, it is not possible to automatically adjust a bias according to variations in the characteristics of elements. Therefore, in general, a bias generation circuit is configured by a comparatively complicated circuit that uses many elements including transistors.
For example, Japanese Patent Application Laid-Open Publication No. 2005-94635 discloses a bias circuit of a power amplifier that is configured to prevent a bias current that has been set once from changing due to changes in the ambient temperature.
Thus, because a bias generation circuit has a complicated circuit configuration that includes transistors, in some cases a bias generation circuit becomes a major noise source within the overall circuit. A transistor generates flicker noise (1/f noise) that is inversely proportional to the frequency. Due to the influence thereof, flicker noise also mixes with a bias voltage (current) that is generated by a bias generation circuit, and adversely affects the operation of a circuit to which the bias is supplied. For example, when noise mixes with a bias current of a voltage controlled oscillator, the oscillation frequency of the voltage controlled oscillator fluctuates.
This kind of bias voltage (or current) noise is generally removed utilizing a low-frequency eliminating filter (low-pass filter). However, since the level of flicker noise increases as the frequency decreases, a low-pass filter with a low cut-off frequency is required to eliminate flicker noise. A low-pass filter includes a resistive element and a capacitive element, and in order to lower the cut-off frequency of a low-pass filter it is necessary to increase the size of these elements when forming the integrated circuit.
That is, a low-pass filter with a large area is required in order to reduce noise occurring in a bias voltage (current), which leads to an increase in the size of an LSI and an increase in costs.